Heat exchanger apparatus with a novel by-passing arrangement for shellside flow



Dec. 24, 1968 R. D. SMITH HEAT EXCHANGER APPARATUS WITH A NOVEL BY-PASSING ARRANGEMENT FOR SHELLSIDE FLOW 2 Sheets-Sheet 1 Filed Nov. 30, 1966 FIG.'

[la I1 l INVENTOR ROBERT DILMAN SMITH ATTORNEY.

3,417,812 -PASSING 2 Sheets-Sheet 2 otoo ocorotnoc E228 3o. 2 2 ouroufnw 535.25 53:5." .F

r8.2 oznEu conndnm 22 .8 22ml@ :8B an.: EEESEaQzouf Dec. 24, 1968 R. D. SMITH HEAT EXCHANGER APPARATUS WITH A NOVEL BY ARRANGEMENT FOR SHELLSIDE FLOW Filed Nov. 30, 1966 United States Patent O 3,417,812 HEAT EXCHANGER APPARATUS WITH A NOVEL BY-PASSING ARRANGEMENT FOR SHELLSIDE FLOW Robert Dilman Smith, Chalfonte, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmingtou, Del., a corporation of Delaware Filed Nov. 30, 1966, Ser. No. 598,070 Claims. (Cl. 165-1) ABSTRACT OF THE DISCLOSURE A tube in shell type heat transfer apparatus having a bundle of highly flexible plastic tubes of low thermal conductivity positioned in a hollow shell with an elongated annular space between the shell and the bundle and having at least one annular baflle mounted in the annular space for selective positioning to achieve an optimum combination of pressure drop and 'heat transfer coecient, the portions of the tubes upstream of the baille provided with means to support them against radial compaction -of the bundle and the portions of the tubes downstream of the baffle provided with means to support them against radial expansion of the bundle.

This invention relates generally to improved heat exchange, or heat transfer, appara-tus and process arrangements; and more specifically to particular heat exchange arrangements involving the use of plastic tubular elements.

As indicated generally in U.S. Patent No. 3,228,456, heat exchange arrangements utilizing plastic tube elements are known and have been found to be advantageous in many applications. Because of the use of plastic tubular elements, a number of special problems have been encountered, not only in designing and producing such arrangements, but also in their operation. Some of these problems involve the relatively low heat transfer coefficients and the relatively high coefficients of thermal expansion possessed by parts made of most plastic materials as compared with parts made of more conventional materials such as metals. Other problems relate to the different properties such as compressive strength, tensile strength and melting points o-f these plastic materials relative to the other materials used. For example, production and operating techniques as well as apparatus designs must be such that the more conventional parts such as those still formed of the usual metallic compositions can be shaped, formed, treated, and used while in cooperative association with the plastic parts to produce and operate economical, reliable, and effective heat exchange arrangements without destroying or damaging the associated plastic parts by application of excessive forces, impacts or temperatures.

It is an object of the invention to provide novel and improved apparatus and process combinations or arrangements having plastic components, especially tube elements, and components made of the materials such as metals, which arrangements perform effectively, reliably, and in addition, not only accommodate but utilize, advantageously, -during their operation, the different properties of the elastic components under the desired operating conditions.

This and other objects have been achieved in applicants improved arrangement which generally comprises process steps and means for accomplishing heat transfer between a rst fluid having a given temperature and a second fluid having a different temperature by passing one of said fluids through the interiors of a compact bundle of a large number of elongated hollow ilexible thin-walled small diameter tube elements said tube elements formed of a material having relatively low heat transfer properties,

3,417,812 Patented Dec. 24, 1968 ice said process lfurther comprising concurrently passing the other of said iluids through an elongated annular zone surrounding and contacting said bundle 4of tube elements and causing said other of said fluids to sequentially engage the exterior of said bundle in longitudinal flow at a number of spaced annular bypass positions along the bundle and at another number of alternate positions between said bypass positions to engage only the tube elements in the interior of the bundle in longitudinal flow, the number and extent of said positions maintained such that, relative to the amount of said other fluid handled, and the desired amount of heat to be transferred, an optimum combination of low pressure drop and high heat transfer conditions is achieved, the heat transfer properties of said tube elements being suticiently low to prevent variations in the flow rate conditions along the bundle from dominantly affecting the overall heat transfer performance of the process over a wide range of operating conditions.

Other objects and advantages will appear from a consideration of the following specifications and claims, taken in conjunction with the accompanying drawings in which:

FIGURE l is a partial longitudinal or side view of a heat exchanger apparatus embodying principles of the invention; with certain parts partially broken away to better illustrate the construction.

FIGURE 2 is a transverse cross-sectional view of the apparatus of FIGURE 1 taken 'at line 2-2 of FIGURE l.

FIGURE 3 is a schematic block diagram showing how the calculated fluid ow losses in heat exchanger performance may be broken down or analyzed.

FIGURE 4 is an enlarged showing of a number of tubular elements of ra heat exchanger illustrating how some of the relationships may be determined for use in determining a bafe arrangement to give optimum heat exchanger performance.

FIGURE 5 is a general schematic showing of a heat exchanger embodying features of the invention illustrating some of the important relationships `and features in determining a baille arrangement to give optimum heat exchanger performance.

FIGURE 6 is an enlarged view of a portion of the apparatus shown in FIGURE 2 illustrating in greater detail the adjustable securing means yfor securing elements 13 to member 10.

One preferred heat exchange apparatus embodying principles of this invention is shown in FIGURES l, 2, and 6. The apparatus shown in these figures comprises an elongated cylindrical casing member 1 formed of a suitable material, such as, for example, steel and provided with anged uid inlet and outlet connections 4 and 5 as shown. A compact elongated bundle (six to ten or more feet in length) of a relatively large number (from about 500 to about 10,000 or more) of small flexible tube elements 11 is positioned as shown inside casing member 1. The bundle of tube elements 11 is positioned in the casing member such that a substantial elongated annular clearance space exists between the outer periphery of the bundle and the inside of the casing member. The Tube elements 11 may be formed of a composition and in sizes disclosed in prior U.S. Patent 3,228,456. The composition preferred is a polyuorinated plastic such as a copolymer of tetrauoroethylene and hexafluoropropylene with a thermal conductivity of about 0.1 B.t.u./ hr./ft./ F. The tube elements in a bundle are preferably in a size range between about 5 and 275 mils outside diameter with a wall thickness between about 0.5 to about 30.0 mils, on centerlines spaced about LOI-2.00 diameters.

The open end portions of tube elements 11 of the bundle lie in a substantially coplanar arrangement (not shown) and are joined in a suitable manner to a header wall member or tube sheet (not shown) at each end of the casing member at the adapter members 6 and 7. Suitable header arrangements are disclosed in U.S. Patent 3,228,456 mentioned above and in prior copending application Ser. No. 389,109 filed Aug. 12, 1964, now Patent No. 3,277,959 in the name of Michael S. Withers.

Conduit element 8 supplies a first heat exchange fluid to the interiors of the tube elements 11 and conduit element 9 at the other end of casing member 1 collects and removes this fluid from the interiors of the tube elements. As indicated in the aforementioned application, Ser. No. 389,109, and in prior copending U.S. application, Ser. No. 408,096 filed Nov. 2, 1964, in the name of Robert J. Moore, the tube bundle may be fabricated by arranging a large plurality of equal length tube elements in a closely spaced parallel side-by-side array, suitably .bonding to said tube elements, flexible narrow spacer tape elements extending transversely across the array at spaced positions thereof, and then winding or rolling up the taped array to form a bundle of desired size with the tape elements securing the tube elements to each other in desired spatial relationship. These tape elements are indicated generally by the reference numeral 12 in FIGURES 1 and 2. The spacing of the tapes along a bundle of tube elements, and the crossing angle at which the parallel tube elements extend are carefully controlled to achieve a taped arrangement or bundle with minimum susceptibility to plugging minimum resistance to flow, low cost, and ease of fabrication. ln a preferred embodiment the spacing between corresponding points on adjacent tape elements was maintained at about six inches with good results. The crossing angle may vary to some extent with the size of the bundle, approaching 90 for the smaller sizes. The operative range of angle lies between about 85" and about 45 with the range of about 80 to about 750 being preferred for optimum results producing a conical tape arrangement in the tube bundle having an included angle of from 150-160. It is pre- Yferred for many applications of the invention that the spacer tape or strip be formed of the same material as the tube elements. The spacer tape should be as narrow and thin as possible within the limitations of the required strength. Suitable tapes have been made with a thickness of 0.018 inch and a width of about 0.5 inch.

The bundle of tube elements 11, when relatively large in size is preferably surrounded, or encased, and engaged by a forminous sleeve member 10 formed of a suitable material substantially rigid and strong such as a steel. A numberl (at each one) of fluid by-pass controlling elements or baflle elements 13, with an annular configuration, are spaced along the bundle in engagement between the interior of the casing member 1 and the encased bundle of tube elements 11 as shown. Batlle or fluid bypassed controlling elements 13 are adjustably secured in desired longitudinal positions along the bundle and casing member -by means of longitudinally protruding lugs 18 on the elements 13 which carry in threaded engagement therewith set, or clamping screws 14 which cooperate, in the embodiment shown, with the foraminous sleeve member 10 to secure the baflle elements 13 in desired position. The construction and dimensions of the casing member 1 and the encased bundle with its baflle elements 13 are controlled such that the encased bundle and baflle elements 13 are first assembled in the desired arrangement and then positioned within the casing member 1 by longitudinally sliding movement from one end thereof. As will be discussed at a later point in more detail, the percent of the length of the casing member 1 between its inlet and outlet connections 4 and 5 which is covered'between or by the baflle elements 13,- thenumber, and positioning of the baflle elements can be selected in accordance with the desired amount of a second heat exchange fluid to be circulated from inlet 4 to outlet 5 through the casing member l outside of tube elements 11 and in accordance with the amount of heat to be transferred between the two heat exchange fluids, to achieve an optimum combination of low pressure drop in the fluid moving outside of tube element 11 in the casing member, with a high overall heat transfer coefllcient. With apparatus of the type disclosed, satisfactory operation is achieved when the optimum combination referred to generally involves an overall heat transfer coefllcient of about B.t.u./hr./ ft.2/ F. and involves pressure drops in the range of about 5 to about 20 p.s.i.g. The high resistance to heat transfer, or in other words, the low heat transfer coefficient of the tube element walls of the specified compositons, is a significant factor in preventing the rate of fluid flow along the bundle from becoming a dominant factor in the heat exchange function of the apparatus Within limits.

The general operation of the apparatus is similar to that of conventional tube and shell heat exchanger apparatus, the first heat exchange fluid at a given temperature being passed through the interiors of the tube elements and the second heat exchange fluid at a significantly different temperature being passed via inlet connection 4 into the casing member 1 surrounding the tube elements and thence out of the casing member 1 by means of outlet connection 5 so that significant heat transfer is accomplished from one fluid to the other through the walls of the tube elements 1:1. The special novel feature of operation is the controlled b-y-passing of certain longitudinally spaced portions of the tube element bundle under the action or effect of the baflle elements '13. As shown in FIGURE 1 each baflle element creates a first zone just upstream of its position in which fluid moving through the casing member from inlet means 4 toward outlet means 5 is caused to move with an inwardlydirected (toward the center of the bundle) component of movement. At the baflle element position the direction of iluid flow is substantially longitudinal along the direction in which the bundle and tube elements extend. Just downstream of each baflle element position is a second zone in which the fluid moving through the casing member is caused to move with an outwardly-directed (away from the center of the bundle) component of movement. With the tube elements made of flexible material the tape elements 12 perform, in the first zones, an additional important function of supporting the tube elements against crushing or flattening under the forces of the inwardly-directed fluid. In addition, with such flexible tubes the foraminous sleeve member 10 performs, in the second zones, an important function of supporting the tube elements against the forces of the outwardly-directed fluid which might otherwise damage the tube elements. By careful control of the number and spacing between the baflle elements 13 the pressure drop of fluid moving through the casing member and the overall heat transfer coefficient of the apparatus can be controlled. The spacing between the baflle elements will control to a considerable extent the intensity of the inwardly and outwardly directed yforces on the tube elements. The flow rate through the casing and baflle element spacing should also be controlled (flow rate low enough and spacing great enough) so that the tube elements are not damaged or flattened by the inwardly or outwardly directed fluid flow notwithstanding the beneficial supporting action of the spacer tape elements and foraminous member.

The principle of the construction and operation which controls the amount of shellside fluid in longitudinal flow relative to the .amount of fluid bypassing a bundle of tubular elements in the disclosed combination is believed to be of significance and applicable whether the tube elements are flexible or not but is limited to applications where the heat transfer coeflicient of the tube element walls is sufficiently low to prevent the fluid flow rate along the bundle from becoming a dominating factor in the overall heat transfer function of the apparatus. Where, as with most plastic materials, especially thermoplastics, llexibility or pliability of the tube elements exists, then the above-described means for maintaining the general position of the tube elements intermediate the ends of the bundle are required.

Determination of a ibafile arrangement for apparatus embodying the invention which arrangement will provide the optimum combination of low pressure drop with high heat transfer coefficient for a -given `amount of fluid to be handled and a given amount of heat to be transferred without damage to the tube element bundle may be accomplished in a number of ways by those skilled in the heat exchanger art.

One way involves, after a general selection of a casing member-tube bundle combination at least adequate to handle the desired volume of fluid per unit time and handle the amount of heat to be transferred per unit time, actual experimentation, on a trial and error basis, with the number and positioning of the baffle elements to determine which arrangement will provide the best overall heat transfer coefficient withY the lowest pressure drop in the shellside uid flow path. It is possible that several arrangements would produce an optimum combination of these factors, in which case the arrangement which is simplest, easiest to fabricate, and lowest in cost would be selected. This usually would mean the arrangement With the smallest number of baffle elements.

Another way involves a corresponding procedure in which the actual experimentation is replaced by corresponding mathematical calculations or operations preferably handled by a computer in which the theoretical performance as to overall heat transfer coefficient and shellside fluid pressure drop is calculated for various possible arrangements as to number and position of the baille elements, and the theoretical performance compared to deter-mine the optimum one using the same criteria discussed in the approach discussed in the preceding paragraph.

In using the second approach certain special relationships and functions which have been determined empiri-V cally have been found valuable. Starting with the wellknown relationship or expression (l) l l l in which U is the overall heat transfer coefficient, h1 is the inside film heat transfer coefficient of the tube elements which is calculable according to known straightforward methods, R is the resistance of the tube element walls and ho is the outside film coefficient of the tube elements which varies as a `function of the effective velocity Vt* of the uid moving along the outside of the tubes (see FIGURE 4); it will be understood that with h1 and R substantially unchanged, the ability to manipulate hg according to this invention involves control of the fiuid flow conditions outside of the tube elements especially effective velocity of the fluid enga-ging the tube elements. For many embodiments of this invention where the shellside fluid is a liquid, the special relationship of this effective velocity Vtl, which affects the heat transfer coefficient, to other 'factors including the bathing arrangement is generally expressed as follows:

in which N is the number of equally spaced annular baffles, `IIN is the fraction of casing member length over which baffling is extended as illustrated in FIGURE 5 and Vnom is the nominal longitudinal velocity as determined by the amount of fluid passing in shellside flow per unit time and the effective cross sectional area through which this fluid must pass in the casing member.

Substituting various values in Expression 2 corresponding to the possible baffling arrangements will provide an effective fluid velocity Vt* value which corresponds for a given allowable pressure drop, to the most desirable, or highest, outside film heat transfer coefficient ho and also the highest overall heat transfer coefficient U0.

As to calculating or establishing shellside fluid pressure drop with applications of this invention, this drop is the summation of several losses or drops as shown generally in FIGURE 3. Three of these losses, A, B, and C are fixed or controlled by total shellside ow and geometrical factors such as inlet connection size and entrance area. Two losses, D and E are produced by viscous shear along the surfaces of the tube elements and by turbulence at the spacer tape elements and are related to appropriate effective velocities Vh* in the classical equations shown.

For many embodiments of this invention where the shellside fluid is a liquid the special relationship of this effective velocity Vhi, which importantly affects the shellside pressure drop, to other factors including the baffling arrangement is generally expressed as follows:

in which N, N, and Vnom have the same definitions indicated in the preceding discussion of Expression or Equation 2.

Substituting various values in Expression 3 corresponding to possible bafiiing arrangements will provide an effective fluid velocity Vh* value which corresponds to a desired or a given shellside pressure drop. The -most simple and economical baffle arrangement is selected which provides the optimum combination of high heat transfer eoefficient and low pressure drop.

A large portion of the demand for the heat exchange apparatus of this invention involves replacement of conventional metal tube units. Because of the adjustable features and operation of the invention this replacement may be accomplished in many cases without requiring other changes in a system in which such replacement is being accomplished. These changes could include new or additional pumps and new piping systems. The adjustable shellside flow arrangement of this invention also offers benefits and advantages in that (l) a tolerance or capability exists, after construction and delivery of a heat exchanger unit (if for example, the process conditions upon installation vary from those initially specified) for quick adjustment or change in the baffling arrangement to obtain optimum performance for the existing conditions; and (2) a potential for process expansion exists where for example an oversized unit is purchased originally in anticipation of future process flow increases, effective shellside fluid velocities can be optimized for initial low flow requirements by increasing baffle element separation and increasing number of baffle elements; and good performance obtained at increased flow requirements by decreasing baffle element separation and decreasing the number of oafile elements.

A series of tests were made with a heat exchanger apparatus embodying features of this invention generally similar to that shown in FIGURES l, 2, and 6, with about 2500 tube elements in a bundle about 6 feet long. The tube side fluid was steam and the shellside uid was water. While maintaining an overall heat transfer coefficient of B.t.u./ft.2/hr./ F. the following flows and pressure drops were observed for the baffle arrangements indicated:

*Infinite number of baffle elements, equivalent to tube element bundle tightly packed in casing member with substantially no clearance between outside of bundle and inside of casing member.

Under these circumstances it will be seen that the ability to adjust the bypassing or baffle arrangement away from the full longitudinal shellside flow conditions of (A) toward those of D makes it possible to maintain the overall heat transfer coeicient lat about 80 B.t.u./ft.2/ hr./ F. with reasonable acceptable shellside pressure drops (i.e., below 20 psi.) up to shellside ows as high as 450 g.p.m. If the full longitudinal shellside ow arrangement of A were the only choice and adjustment of this ow arrangement was not possible, the shellside pressure drop would become prohibitive very quickly. For example, calculated pressure drops for the same total ows mentioned above are indicated as follows:

AP for full longitudinal flow arrangement of (A) This data indicates that, for a given installation requiring heating 450 gpm. with 20 p.s.i. maximum acceptable shellside pressure drop it would be possible to use one of this type exchanger unit the (D) baffling arrangement Whereas four of the same units with the (A) baiing arrangement would otherwise be required.

It is believed to be apparent that a novel and improved heat exchange arrange-rnent has been provided in accordance with the object of the invention.

Although a preferred embodiment of the invention has been described in detail in accordance with the Patent Laws, many variations and modications within the spirit of the invention will be obvious to those skilled in the art, and all such are considered to fall within the scope of the following claims.

What is claimed is:

1. An improved heat exchange apparatus comprising in combination; an elongated casing member provided with a uid inlet and a iiuid outlet longitudinally spaced therefrom, an elongated bundle of a relatively large number of small flexible tube elements of organic polymeric composition the apparent heat transfer coeicient of said tube elements 'being significantly lower than that of any metal said bundle positioned in said casing member such that an elongated annular space exists between the outside of the bundle and the inside of said casing member and in communication with the said inlet and said outlet, at least one annular baffle member cooperating with the bundle and the inside of the casing in substantially fluidtight relationship to divide the annular space into at least two zones, each said baie member positioned at a given point along the bundle of tube elements in the casing member between the inlet and outlet and arranged such that on the inlet side of said baffle member a first fluid ow zone is formed 2in which iiuid iiowing from the inlet along the outside of the tube elements in the bundle toward the outlet is forced to move inwardly between the tube elements of the bundle, and such that on the outlet side of cach said baie member a second fluid ow zone is formed in which fluid moving from the inlet toward the outlet along the outside of the tube elements in the bundle is forced to move outwardly between the tube elements of the bundle, said apparatus further comprising spacer lmeans cooperating with the tube elements in the tirst zone to maintain said exible tube elements in spaced apart relationship to each other against the force of the inwardly moving fluid, and restraining means cooperating with tube elements of the bundle in said second zone to maintain the exible tube elements in predetermined space positions relative to each other against the force of the outwardly moving iiuid, each said bale member being adjustably mounted in said casing member for selective relative positioning along the length of the casing member and bundle to control fluid flow in the casing member along the outside of the tube elements in order to achieve an optimum combination of high heat transfer and low pressure drop in the casing for varying magni- 8 tudes of flow through said casing member and without iiattening or damaging of the tube elements under the action of the inwardly and outwardly moving uid, said restraining means comprising an open annular framework assembly surrounding and cooperating with the bundle of tube elements in the second zone, said assembly comprising a foraminous sleeve member extending along, surrounding, and engaging the bundle in said second zone.

2. The improved combination of claim 1 in which the spacer means comprises at least one elongated spacer strip interleaved between said tube elements in a predetermined pattern in a substantially transversely extending Section across said bundle, said pattern being conical with an included angle of from -160.

3. The improved combination of claim 1 in which said tube elements are formed of a polyuorinated plastic material with size between about 5 and about 300 mils outside diameter with wall thickness between about 0.5 and about 30 mils and the number of tube elements in a bundle is between about 500 and about 5000.

4. An improved heat exchange apparatus comprising in combination, an elongated cylindrical casing member provided with a uid inlet and a uid outlet longitudinally spaced therefrom, an elongated ybundle of tube elements, said tube elements formed of a material having a thermal conductivity no greater than 10.0 B.t.u./hr./ft./ F., said bundle positioned in said casing member such that an annular space exists between the outside of the bundle and the inside of said casing member and in communication with the said inlet and said outlet, at least one annular baflie member cooperating with an extending around the periphery of said bundle and Cooperating with the inside of the casing to divide the annular space into at least two zones, each said baiile member positioned at a given point along the bundle of tube elements in the casing member between the inlet and outlet and arranged such that on the inlet side of each said baie member a irst fluid flow zone is formed in which uid flowing from the^inlet along the outside of the tube elements in the bundle toward the outlet is forced to move inwardly between the tube elements of the bundle, and such that on the outlet side of each said baffle member a second uid ow Zone is formed in which uid moving from the inlet toward the outlet along the outside of the tube elements in the bundle is forced to move outwardly between the tube elements of the `bundle, said apparatus further comprising positioning means c0- operating with the tube elements to maintain said tube elements in said first zone in desired relationship to each other against the force of the inwardly moving fluid tending to compact said bundle, and said positioning means also cooperating with tube elements of the bundle to maintain the tube elements in said second zone in desired positions relative to each other against the force of the outwardly moving uid tending to expand the bundle, each said baiile member lbeing adjustably mounted in said casing member for selective relative positioning to a predetermined position along the length of the casing member and the bundle to selectively vary and control iiuid flow in the casing member along the outside of the tube elements in order to achieve an optimum combination of high overall heat transfer coeicient and low pressure drop in the casing for any desired magnitude of ow through said casing member and any desired total amount of heat to be transferred.

5. An improved process for accomplishing heat transfer between a first fluid having a given temperature and a second uid having a lower temperature, said process comprising passing one of said iiuids through the interiors of a compact bundle of a large number of elongated hollow flexible thin-walled small diameter tube elements said tube elements formed of a material having predetermined relatively low heat transfer properties, said process further comprising concurrently passing the other of said fluids longitudinally through an elongated annular 9 zone surrounding and contacting said bundle of tube elements and causing said other of said ilnids to sequentially engage the exterior of said bundle in longitudinal ow at a number of longitudinally spaced annular bypass positions along the bundle and at another number of alternate positions between said bypass positions to engage only the tube elements in the interior of the bundle in longitudinal ow, the number and extent of said positions maintained such that, relative to the amount of said other uid handled, and the desired amount of heat to be transferred, an optimum combination of low pressure drop and high heat transfer conditions is achieved, the predetermined heat transfer properties of said tube elements being sn'iciently low to prevent variations in the References Cited UNITED STATES PATENTS 1/1950 James 16S-159 10/1966 Withers e 165-159 ROBERT A. OLEARY, Primary Examiner.

CHARLES SUKALO, Assistant Examiner.

U.S. Cl. X.R. 16S-160, 162, 175 

